1. Technical Field of the Invention
The present invention relates to modem technology and, in particular, to an echo canceler for use within a modem.
2. Description of Related Art
The physical media for carrying telephone communications to and from a residence typically comprises a twisted pair of copper wires. The twisted pair supports a communications bandwidth of several mega-hertz. Only the lower portion of that supported bandwidth from 300-4,000 Hz (i.e., the voice band), however, is typically used in conventional telecommunications networks to carry user telephone and data communications. The remainder of the available bandwidth on a twisted pair has not been used for data communications due primarily to a lack of support at the network and concerns over attenuation. Notwithstanding the foregoing, there would be an advantage to separating data from voice communications and moving the data communications over the twisted pair out of the voice band. This would allow for the use of a single twisted pair media to simultaneously carry both a voice and data communication.
Asymmetric Digital Subscriber Line (ADSL) provides a new modem technology that converts an existing twisted pair line into a plurality of access paths for multimedia and high speed data communications. ADSL transmits more than six megabits per second on the downlink to the user, and six-hundred forty kilo-bits per second or more in both directions (up and down), while simultaneously allowing conventional use of the twisted pair to carry user voice communications within the voice band. Using ADSL technology, the existing telecommunications network can be transformed to fully and efficiently support voice, text, file transfer, high resolution graphics and multimedia service delivery to every user.
Referring now to FIG. 1, there is shown a block diagram of a telecommunications network 10 supporting the use of ADSL technology. An ADSL modem 12 is connected at each end of a twisted pair telephone line 14. At one end of the line 14, a first modem 12(1) interfaces with customer premises equipment 16 comprising, for example, a conventional telephone set or a personal computer. At the other end of the line 14, a second modem 12(2) interfaces with the conventional core telecommunications network 18. The pair of modems 12 function to create three channels for the communication of information over the twisted pair line 14. The first channel is a high speed downlink channel suitable for the communication of data (such as a file transfer) to the user. The second channel is a medium speed duplex channel suitable for the communication of data between the customer premises equipment 16 and the conventional core telecommunications network 18. Each of the first and second channels may be sub-multiplexed as needed to form multiple, lower rate, channels. The third channel comprises a plain old telephone service (POTS) or integrated services digital network (ISDN) channel suitable for supporting conventional voice and data services. The POTS/ISDN channel is typically split off at the customer premises equipment 16 by filters in the modem 12 to ensure uninterrupted provision of conventional voice and data services in the event the modem 12 should fail.
To create the multiple channels, the ADSL modem 12 divides the available bandwidth of the twisted pair telephone line in one of two ways. First, as illustrated in FIG. 2A, frequency division multiplexing (FDM) is used to assign one sub-band to carry uplink data (from the customer premises equipment 16) and another band to carry downlink data (to the customer premises equipment 16). Second, as illustrated in FIG. 2B, the bands for uplink and downlink data communication are assigned to partially overlay one another, and local echo cancellation is used at each end of the twisted pair line 14 to separate the two bands from each other and recover the transmitted information. In either case, the ADSL modem 12 splits off the four kilohertz voice band at the DC end of the available bandwidth to support conventional communications services.
It is recognized that twisted pair telephone lines 14 having a long length significantly attenuate communicated information signals at around one megahertz (i.e., at the outer edge of the bandwidth used by ADSL) by as much as ninety dB. It is accordingly necessary that the transmitter in the modem 12 transmit its high speed downlink channel or medium speed duplex channel signals over the line 14 with a relatively high signal power. The increased transmit signal power, however, does create some difficulties such as an unwanted local echo at the modem 12 which may adversely affect the modem""s ability to detect receive signals while in a duplex operational mode.
Provision of an echo cancellation functionality within the modem 12 would adequately address this difficulty and substantially eliminate the local echo. A block diagram for a traditional, prior art, fully digital domain adaptive echo canceler 20 for use within the ADSL modem 12 is shown in FIG. 3. The echo problem arises at the twisted pair line interface 22 for the modem 12 where a portion Ve (called near end or local echo) of the transmit signal Vtx appears in the received signal Vrx (due, for example, to leakage across the interface 22 or reflection/impedance mismatch along the twisted pair line 14). In the echo canceler 20, the generation of the echo cancellation signal Vexe2x80x2 (see, at the output of adaptive filter 24Y and cancellation operation itself (see, at summer 26 are performed directly and wholly in the digital domain 28.
Due to the high losses experienced with transmission of signals over the twisted pair line 14, the magnitude of the receive component Vrx of the composite signal Vrx+Ve may be very small relative to the magnitude of the echo portion Ve. This implies that the modem 12 components in the receive channel 30, especially those components located within the analog domain 32 (such as the receive amplifier, filters and analog-to-digital converter), must be capable of handling signals with a relatively large dynamic range (in the order of 80-90 dB). While it is possible to design and build such a device, its design and implementation present quite complicated and expensive propositions that detract from the advantages of ADSL technology. Still another limitation of a fully digital echo canceler is the substantial computational complexity of the adaptive filter 24 (comparable of that of an entire modem), which arises due to the high complexity of the echo path 33 transfer function that incorporates transmit, echo and receive chains. There is accordingly a need for a less complicated and expensive echo cancellation functionality for use within ADSL modems.
A modem including an echo cancellation functionality includes both a digital processing domain and an analog processing domain. A digital transmit signal is processed through a transmit channel and output over a twisted pair, with a portion of the transmit signal appearing within a received signal as an unwanted echo component. The echo cancellation functionality taps the digital transmit signal from the transmit channel in the digital processing domain. The tapped signal is processed in an echo channel through an adaptive filter (whose transfer function substantially matches an echo transfer function) in the digital processing domain to generate a digital echo cancellation signal. The digital echo cancellation signal is then converted to an analog echo cancellation signal and subtracted in the analog processing domain from the received signal (which includes the unwanted echo component) to substantially cancel the echo before further processing in a receive channel of the modem.
The echo cancellation functionality is implemented within the modem to provide two operational modes. In a first (training) mode of operation, the echo cancellation functionality configures the transmit channel, echo channel and receive channel of the modem to either by-pass certain modem components having transfer functions that would otherwise introduce amplitude and phase distortions in an error signal (comprising the echo component minus the echo cancellation signal), or to define some transfer functions in the modem, in order to tune the transfer function of the adaptive filter to substantially match the echo transfer function. Following completion of the training mode of operation, the echo cancellation functionality configures the modem into a second (operating) mode of operation to support information transfer to and from the modem over the twisted pair.
The adaptive filter may comprise any tunable finite or infinite impulse response filter including known least mean square, Kalman and recursive mean square types. Preferably, however, the modem comprises a least mean square type finite impulse response adaptive filter that may also contain a predistortion function in accordance with the present invention. The adaptive filter accordingly includes a first n-tap delay line and a recursive tap weight update section in accordance with conventional filter design. To implement the predistortion function, the adaptive filter further includes a second n-tap delay line with a filter block whose transfer function substantially matches a combined transfer function for an adaptation loop of the modem comprising the echo channel and the receive channel. The outputs of the second n-tap delay line then generate a predistortion vector for input to the recursive tap weight update section of the adaptive filter.